Ocean Acidification

• A rapid synchronous negative δ¹³C_org and δ¹³C_carb excursion occurred in the early Kungurian.

• The excursion can be traced in marine and terrestrial sections worldwide.

• We propose a model based on carbon mass-balance calculations and geological data.

• The concurrent volcanism, methane leakage, and wildfires might have steered the excursion.

Abstract

The Kungurian (late Cisuralian) is associated with elevated atmospheric pCO₂ levels and numerous substantial changes in the ocean and terrestrial systems, but carbon cycle perturbations through this time interval are poorly understood. Here, three well-dated organic carbon isotope (δ¹³C_org) curves from South China, paired with their respective carbonate carbon isotope (δ¹³C_carb) data published before, are presented for the first time. Their comparison demonstrates that a prominent, rapid, and synchronous negative excursion in δ¹³C_org and δ¹³C_carb occurred in the early Kungurian (the herein proposed KCIE event dated to ca. 277.9–277.4 Ma). A comparable shift is observed in other deeper-water marine sections in South China and two terrestrial sections in North China and eastern Australia. Based on carbon mass-balance calculations and a review of concurrent geological records, we propose that the KCIE event was driven by an intensive volcanism of a large igneous province in northern Gondwana, dissociation of voluminous methane hydrates induced by this volcanism, and the ensuing worldwide conflagrations. Predicted elevated production of greenhouse gases involved would have warmed the Earth and further resulted in ocean stagnation, anoxia, and acidification, all of which are in conformity with the documented sedimentological, geochemical, and palaeontological data. This major anomaly in the carbon cycle in the late Cisuralian is considered here to play a key role to promote the transition of the Earth to a greenhouse mode.